Sewer diversion, saddle connection & remedial repair work

Overview

We first engaged with the University on 14th June 2023, when it was explained that the existing private drainage system was experiencing issues. Following a CCTV survey, it was confirmed that the problem was caused by a collapse in the 150mm carrier drain.

This was now being jetted every other day to maintain flow for the University and businesses running every day. After many visits and meetings on site with the University, along with additional background information from the client regarding previous works carried out by others, we began to understand the issues and identify the main cause of the collapse.

Reviewing the existing drainage drawings and photos taken during previous works, we soon realised the collapsed main was installed below an attenuation tank, meaning in storm conditions, when the tank fills, it puts a lot of pressure on the pipe, causing it to collapse.

Further damage was caused by previous contractors during the last repair. They had restored the collapse pipe but in doing so they excavated/pulled out the end of the attenuation tank, and this wasn’t reinstated correctly when the work was complete, causing water to leave the tank and flow into the surrounding areas. This caused subsidence.

Scope

Phase 1 west section:
Diverting an existing pipe run onto the existing main sewer at 3.5m depth within the public highway (Penrose Way) excavation via open cut.

Phase 2 east section:
The replacement of the existing pipe run between two existing chambers, depth not exceeding 3m, via open cut.

Phase 3:
The replacement and repair of the existing surface water attenuation tank depth not exceeding 2m via open cut. (damage by previous contractors)

Phase 4:
An area approximately 75m² of paving had dropped east side of the University this area had to be lifted and relaid at the correct finished levels.

Phase 5:
1050mm surface water carrier drain, which needed repairing (damage by previous contractors).

Phase 6:
Renewal of the Qmax slot drain at a depth not exceeding 0.8 metres, along with the replacement of a Power/COMMs chamber to a depth of 1.5 metres.

Key Requirements & Challenges:

• Pedestrian Management:
  Being a University, we had plenty of students coming in and out of the building daily, so planning combined with good communication with the client was a priority. A temporary footpath was put in place with extra signage to guide the pedestrians in the right direction & destination.
• Over Pumping:
  Temporary over-pumping was installed during our time on site, so it was important to maintain the flow not only for us to carry out the remedial work required, but to keep both the University & small business open. All equipment installed was super silent, which meant noise levels were kept to the absolute minimum.
• Depth of Excavation (3.5m):
  Excavated using a 5-tonne machine with safety and protection measures in place to ensure stability and to protect the existing infrastructure, this machinery can only be operated by a fully qualified plant operator experienced in deep drainage.
• Precise Connection Method:
  The existing 450mmØ foul water sewer was cored drilled carefully removing a section off the shoulder of the pipe, once removed, a new saddle was installed, it was then sealed with sand, cement and a Sika 4a water stop.
• Reinstatement to Specification:
  • Cut edges, drill bar, A393 mesh poured C40 concrete.
  • Existing slabs replaced.
  • Finished with a final jet down.

At Premier Drainage Solutions, we do not rely on temporary reinstatements. We believe in completing work permanently and to the highest standard, preventing future disruption and repeat visits.

Outcome:

• Sewer diversion and connection completed to Thames Water standards
• Full reinstatement carried out to specification
• Project delivered safely with approved plan and programme for works
• Client received a permanent, long-term solution